The present invention is generally related to an apparatus and process for removing contamination in various phases from contaminated soil and groundwater. More particularly, the present invention is related to a soil and groundwater decontamination system and method for decontamination of soil and groundwater by reaction and extraction.
Various organic contaminants can be found in a contaminated subsurface zone, including the groundwater and soil, such as hydrocarbon compounds including volatile organic compounds (VOCs), semi-volatile organic compounds, and nonvolatile materials, and the like. Sources of surface and subsurface contamination are numerous, for example, leaking underground storage tanks, industrial, and manufacturing operations, chemical storage in process areas, chemical spills, waste disposal areas, etc. Among common contaminants from these sources are petroleum hydrocarbons, such as benzene, toluene, ethylbenzene, xylene, gasoline, diesel fuel, fuel oil, jet fuel, and others. Contaminants can exist in subsurface soil and in groundwater, below the water table, in various phases as discrete substances and mixed with and/or dissolved in groundwater and soil gases. Such contaminants can occur in the vapor phase in the vadose (unsaturated) zone, in the free (separate) phase floating on top of the groundwater (or dense non-aqueous phase liquid (DNAPL) at the base of an aquifer), dissolved phase in the groundwater, and in the absorbed phase in the unsaturated (vadose) zone and saturated groundwater zone below the water table.
A number of techniques are known for removal of soil contaminants and remediation of effected soil. One such technique involves the excavation and treatment of the soil on- or off-site by means such as incineration, chemical treatment, or biological treatment. However, when soil contaminated with volatile organic compounds is excavated, up to about 90 percent of the contaminants may volatize to the atmosphere. Also, there exists concerns regarding landfilling of such materials.
Another technique involves saturating the contaminated soil with water in-situ (soil flushing), causing the contaminants to be slowly leached from the soil by the water. The contaminated water can then be removed. Other common methods include groundwater pump and treat and air sparging.
Vacuum extraction techniques have also been proposed for removing VOCs from soil. Known soil vapor vacuum extraction provides an apparatus and process by which volatile vapors may be extracted from the soil through subsurface vacuum application above the resting water table, thereby extracting vapors present in the vadose zone. There are many problems associated with the soil vapor vacuum extraction process. When a single tube extraction well or well casing is disposed to a depth below the water table surface and a vacuum is applied at the unsubmerged end of the well casing, the resting water table surface can become dynamic resulting in the water table surface moving upward in the area adjacent the well casing, essentially forming a peak and resulting in "upwelling." This upwelling can cause floating contaminants just above the water table surface (in the capillary fringe where up to 80 or 90 percent of the contaminants can reside) to float or gravitate away from the well casing, resulting in a less treatable capillary fringe and the undesired consequence of migration of the floating contamination. Further, the upward movement of the water table surface adjacent the well casing can effectively "seal" the extraction well and prevent the well casing from ingesting the air necessary to maintain the vacuum for extraction.
Dual-phase extraction (also commonly referred to as multi-phase extraction) is yet another technique for removing vapor phase adsorbed phase, liquid phase and dissolved phase VOCs and semi-volatile organic compounds from impacted soils and ground water. Known dual-phase extraction provides an apparatus and process by which volatile vapors and liquids may be extracted from the soil. There are two primary methods known for such dual-phase extraction.
In a first known method of dual-phase vacuum extraction multiple pumps are employed to extract vapor and liquid phases. As such, in addition to the use of the vacuum extraction to extract vapors, a downhole mechanical pump is also introduced into the contaminated soil to extract contaminants present in the difficult to treat saturated zone. While this technique extracts both liquid and vapor phases, it does so in separate streams and through the use of two distinct steps or operations, thereby being less efficient and requiring more resources to be devoted to such a clean-up than where the liquid and vapor phases can be extracted simultaneously and in the same step or operation. An example of such an apparatus and method can be found in U.S. Pat. No. 5,615,974 (Land, et al.)
In a second known method of dual-phase extraction a single pump system is employed to extract vapor and liquid phases. As such, the vacuum extraction provides for the simultaneous removal of the liquid phase and gas phase in one common integrated stream through one common conduit. This process will hereafter be referred to "integrated dual-phase extraction." Such integrated dual-phase extraction can generally be achieved by inserting at least a portion of an extraction well below the resting water table surface of the contaminated subsurface zone.
An example of an integrated dual-phase extraction process and apparatus are disclosed in U.S. Pat. No. 5,050,676 (Hess, et al.), the disclosure of which is herein incorporated by reference. Disclosed is integrated dual-phase extraction for removal of contaminants in the soil and groundwater where the contaminants are in the dissolved phase, liquid phase, and the vapor phase simultaneously in one common integrated stream. The apparatus disclosed includes a vacuum generating device connected to one end of a well casing that is disposed in a first borehole. At least one air inlet well is located in a second separate borehole. The well casing is a single tube configuration.
Another example of an integrated dual-phase vacuum extraction process and apparatus is disclosed in U.S. Pat. No. 5,172,764 (Hajali, et al.), the disclosure of which is herein incorporated by reference. Disclosed is an apparatus and process for removing contaminants from a contaminated subsurface zone when those contaminants are in the absorbed phase, dissolved phase, liquid phase, and/or the vapor phase. The disclosed vacuum system includes a dual pipe configuration having a perforated well casing inside of which is situated a drop tube. The inner drop tube has an opening and the well casing has an air inlet at or near the ground surface. The air inlet permits the introduction of air at any desired pressure, including reduced pressures, atmospheric pressure, or increased or forced pressures into the well casing. The air inlet may also be used to introduce other chemicals or materials into the well casing. Substances introduced by the air inlet into the well casing pass downwardly through the well casing. A vacuum pump is connected to an end of the extraction pipe that remains above ground level, thereby drawing contaminants from the saturated and/or unsaturated (vadose) soil surrounding the perforated region of the well casing through the perforations and into the vacuum extraction pipe, then upwardly with the liquid and vapor mixture through the drop tube. Substances introduced by the air inlet into the well casing pass upwardly with the contaminants through the drop tube. The drop tube draws contaminants from the soil and groundwater in the vapor, dissolved, and liquid phases concurrently in a single step and operation.
The use of a physical reaction, a chemical reaction or a combination thereof for treatment of waste water and petroleum contaminated groundwater is generally known to those skilled in the art. Specifically, the use of chemical oxidation for treatment of wastewater and petroleum contaminated groundwater has been studied extensively and implemented in the wastewater and environmental industries. This oxidation process has been carried out using numerous oxidizing agents, for example, ozone, hydrogen peroxide (H.sub.2 O.sub.2), ozone in conjunction with ultraviolet light, etc.
Fenton's reaction was initially identified approximately 100 years ago and is a process that has been utilized in the wastewater treatment industry for a number of years. The reaction, when carried to completion, simply involves the use of hydrogen peroxide (H.sub.2 O.sub.2 --10-50% concentration) in the presence of a catalyst (e.g., a metallic salt such as iron sulfate (FeSO.sub.4), which in many instances in naturally occurring in the soil) to form hydroxyl radicals (OH.cndot.) that have the potential to ultimately reduce hydrocarbons to carbon dioxide and water. The process is simplified below: EQU H.sub.2 O.sub.2 +catalyst+hydrocarbon.fwdarw.CO.sub.2 +H.sub.2 O
This equation greatly simplifies the intermediate steps of this chemical reaction. It is also noted that there are several different places on organic compounds which the hydroxyl radicals can attach, hence, many different (intermediate) products can be formed during the course of this reaction. Oxidation reactions during Fenton's reaction are complex and it is difficult to predict which intermediate products will be formed. This reaction is most "efficient" at low pH ranges (pH of .about.3), which is generally not practical to create in the subsurface. The low pH requirement of optimum Fenton's reaction appears to be related to the need for soluble iron in the system and the reducing conditions to regenerate iron (II). Richard J. Watts, Hydrogen Peroxide for Physiochemically Degrading Petroleum-Contaminated Soils, Remediation, Autumn 1992.
Hydrogen peroxide in this process is utilized in a different fashion when compared to its typical use in the environmental restoration industry (i.e. as an oxygen source to enhance aerobic biodegradation). This same benefit exists in the Fenton reaction process, however, the primary use of hydrogen peroxide is to generate the hydroxyl radicals, a very powerful (second only to fluorine gas in oxidation potential) and effective nonspecific oxidizing agent which vigorously reduces hydrocarbon chains and rings to carbon dioxide and water when carried to completion.
Additionally, in-situ treatment of a contaminated subsurface zone via physical reaction caused by surfactants and co-solvents is generally known in the art. Although, the use of surfactants and co-solvents for the enhanced recovery of crude oil has been in use for many years, the utilization of surfactants and co-solvents for the enhanced recovery of organic contaminants is a relatively new application. A surfactant (surface acting agent), is a chemical compound that is both hydrophobic ("water fearing") and hydrophilic ("water loving") and has the potential of altering the properties of fluid interfaces. The hydrophobic portion of the surfactant is typically a long hydrocarbon chain, while the hydrophilic portion of the surfactant often includes anion or cations. Surfactants are typically highly soluble in water due to the hydrophilic portion of the surfactant. When a surfactant is introduced in subsurface environments where organic contamination resides, the hydrophobic portion of the surfactant may be attracted to the organic contaminant, while the hydrophilic portion of the surfactant will be oriented toward the water phase, resulting in the accumulation of surfactant monomers at the organic contaminant/water interface or the water/air interface. The introduction of surfactants into the subsurface can achieve two different results-enhanced mobilization of the organic contaminant, and enhanced solubilization (into water) of the organic contaminant, as described below.
Co-solvents are chemical agents, typically alcohols, that can be either utilized to enhance the performance of surfactants, or by themselves to enhance organic contaminant aqueous mobilization or dissolution, similar to that of the surfactant.
Enhanced mobilization of an organic contaminant results where the surfactant reduces the organic contaminant/water interfacial tension. This reduction in interfacial tension diminishes the influence of capillary forces, which are responsible for the retention of the organic contaminant in its residual (adsorbed) and liquid (separate) phases. Physical mobilization of the organic contaminant occurs when sufficient lowering of the interfacial tension between the organic contaminant and water is present.
With respect to enhanced solubilization of the organic contaminant into water, the aqueous solubility of the organic contaminant is increased, thereby "dissolving" the residual (adsorbed) phase and/or liquid (separate) phase organic contaminant into water.
Prior uses of surfactants and co-solvents for subsurface cleanup of organic contaminants have typically involved the injection of these fluids into the subsurface only, or in conjunction with a groundwater withdrawal system using a downhole mechanical pump (i.e. "pump and treat" technology).
However, treating contaminated soil and/or groundwater purely by a reaction process does not address the issue of the need to capture the off-gases and fugitive vapors which result from the reaction (particularly, the Fenton's reaction) or from the undesired displacement of fugitive vapors from the injection (positive pressure) process. Such off-gases and vapors may find pathways of escape, for example, in existing sewer lines, which can result in an unsafe build up of pressure and/or hazardous/explosive vapors underground, creating a safety hazard. Additionally, pure reaction treatment can suffer from insufficient distribution of the chemical product (i.e. the oxidizing agent, surfactant and/or co-solvent) throughout the contaminated subsurface zone.
Further, a single tube configuration for dual-phase extraction can be limited in its use depending on depth of application and porosity of the contaminated subsurface zone in which the extraction is to be used.
Thus, a heretofore unaddressed need exists in the industry for an improved soil remediation system and/or process to address at least the aforementioned deficiencies and inadequacies.